Mater artium necessitas

Archive for February, 2009

After suffering broadband trouble for the past 9 months, including interruptions that lasted a few days, I decided to get an additional line installed by a different ISP.
I could have bought one of these multi-WAN devices but decided against it for a couple of reasons: I like a challenge and I wanted to achieve a particular setup that I wasn’t sure could be answered by off-the-shelf products (for a reasonable price that is).

This long article is fairly detailed but if your setup is similar it should be enough to get you going quickly.

The basic setup

Notable things

CYBR, a standard DSL line with a fixed IP 111.99.88.77 allocated through PPPoE.

HKBN, a standard 100Mbps line with a fixed IP 30.40.50.62.

The network is split into different zones:

the Internet zone, connected to our Firewall through interfaces eth0 (ppp0) and eth1.

a Firewall zone, delimited by the firewall system itself

a DMZ zone connected through interface eth2 for the servers we want to make visible from the Internet.
The DMZ has its own private subnet delimited by 192.168.254.0/255.255.255.0.

a LAN zone connected through interface eth3 so local computers can access the Internet and be protected from it.
The DMZ has its own private subnet delimited by 192.168.0.0/255.255.255.0.

Objectives

What we want from our setup:

our firewall protects our DMZ and LAN from unwanted access.

our win server can host websites or other services.

our linux server can handle receiving and sending email or other services.

our firewall can handle incoming traffic from either ISP.

our firewall can load-balance local outgoing traffic across both ISP.

If one line fails, incoming traffic switches to the other line.

If one line fails, outgoing traffic switches to the other line.

Eventually, we want both the linux and win servers to be able to host different websites and we want the firewall to send incoming requests to the right server.

In this first article, I’ll present the setup for items 1-5.
The remaining topics will be the subject of subsequent articles of their own.

Technologies

The firewall is our primary subject. What is being discussed here is pretty much distribution-independent and should work on all flavours of Linux.

OS on the firewall system

I chose CentOS on the firewall.
Being an almost byte-for-byte identical copy of RedHat Enterprise Linux, all configuration will be identical on RedHat and its derivatives such as Fedora.

Firewall software, first try

When my firewall needs are simpler, I use the Stronger IP Firewall Ruleset from the Linux IP Masquerade HOWTO.
I started to modify the script to adapt it to my new Multi-ISP setup but things got complicated once I needed to debug routing tables.
I got it 80% of the way but tracing network connections and packet routing is complicated and time-consuming.
After a couple of days of peering into log files and wireshark capture screens, I gave up manual configuration and decided to go with something else.

Firewall software, final

The product I chose in the end is shorewall: it’s a very flexible firewall system that create the necessary iptable rules and configure most of the routing needs to properly handle complex network setup.
Shorewall is Open Source, very stable, has been out for a long time, is actively maintained and has lots of excellent documentation and examples.

Things to know

Before we get into the meat of the article, you should brush up on the following topics:

You have some knowledge of Linux system administration, know how to configure network connections, know how to enable/disable/stop/start services, able to edit config files.

Networking: you should know what a netmask is, what a gateway is, what a subnet is and have a passing understanding of IP classes, IP notation, what ports are for, what’s the difference between the tcp, udp, icmp protocols, what Dynamic Port Forwarding (DNAT) is, what Network Address Translation (NAT) is, what masquerading means.

Some basic understanding of DNS and local host name resolving (using host.conf and resolv.conf)

Some basic knowledge of what routing is for and how it works.

Some knowledge of how the linux kernel handles network packets (NetFilter, basics of iptables).

You don’t need to be a specialist in any of these areas but any knowledge helps.
I’m far from being well versed into Netfilter and routing, it’s not often that I have to deal directly with these topics, but brushing up on these topics helped.

Things to read

Shorewall has very extensive documentation. So much so that it can be a bit daunting, not knowing where to start.
I found the following documents helpful to get me started:

Disable firewall

Disable the current firewall, for instance using the system-config-securitylevel helper tool.
Be careful if you’re directly connected to the Internet, you will be left without protection!
You can actually wait until shorewall is properly configured to disable the firewall.

Shorewall configuration

Shorewall uses a set of simple configuration files, all located under /etc/shorewall/.
For exact detail of each configuration files, have a look at the list of man pages.

Zones

zones are probably the simplest configuration file.
Details in the zones man page.
Here we just name the various zones we want our firewall to handle:

Note that for our net zone, we list the 2 interfaces connected to our ISPs.
If you’re using PPPoE to connect, don;t use the interface name eth0 but use ppp0 instead.

Policy

The policy file tells shorewall which default actions should be taken when traffic is moving from one zone to another.
These default actions are taken if no other special action was specified in other configuration files.
View the policy file as a list of default actions for the firewall.
Details about this configuration file as in its man page.

Traffic from one zone to another needs to be explicitely ACCEPTed, REJECTed or DROPped.
For instance, loc net ACCEPT means that we allow all traffic from our local LAN to the Internet, while net all DROP means we don’t allow incoming traffic from the internet to anyone (remember this is the default action, in most cases we will override this for specific types of traffic in the rules file).
When we set the default action to DROP, we can tell shorewall to keep a trace of the details in the /var/log/messages log.

Providers

The providers file is generally only used in a multi-ISP environment.
Here we define how we want to mark packets originating from one ISP with a unique ID so we can tell the kernel to route these packets to the right interface.
Not doing this would get packets received from one interface to be routed to the default gateway instead.
The details of this configuration file are explained in the providers man page for it.

Note that the DUPLICATE columns tells shorewall that it should make a copy of the main default routing table for this particular routing table (called CYBR or HKBN depending on which ISP we refer to).
Packets are marked with number 0x1 or 0x2 so we can distinguish them during their travel through the system.
For PPPoE connections, don’t specify a GATEWAY since it’s most likely that your ISP didn’t give you one.

The most interesting part of this file are the OPTIONS: track means that we want the packets to be tracked as they travel through the system; balance tells the kernel that we want traffic coming out to be spread over both interfaces.
Additionally, we want HKBN to receive more or less 5 times more traffic than CYBR (note that this has no effect on reply packets).

The COPY columns will ensure that the routing tables created for CYBR and HKBN are copied for each internal interface, so our eth2 and eth3 interfaces know how to route packets to the right ISP.

Route Rules

For our purpsose, the route_rules file only describes how traffic should be routed through one or the other ISP we set up in /etc/shorewall/providers.
Details are in the route_rules file man page.

Here we simply say that all traffic through the CYBR table should be sent to ppp0.
The PRIORITY is an ordering number that tell shorewall to consider this routing rule before it marks the packets. Since we know the packets originated from ppp0 or eth1 we don’t really need to mark them.

Masq

The masq file will contain the masquerading rules for our private interfaces: in essence, we want traffic from the local LAN and DMZ to be hidden behind our limited number of external IPs.
See the masq manpage for all the details.

The first part ensures that the traffic coming out of our public interfaces but originating from the other is actually rewritten as originating from the right IP for the interface.
This ensures that packets leaving eth1 for instance don’t come out with the wrong source address of the other interface.
The second part of the ensures that packets from our LAN or DMZ leaving either public interfaces are doing so with the right IP address, so traffic from my desktop going through ppp0 for instance, will have its source address as 100.90.80.70.

Rules

This is the main file where we tell shorewall our basic configuration and how we want packets to be handled in the general case.
The /etc/shorewall/rules file contains the specific instructions on where to direct traffic that will override the default actions defined in the /etc/shorewall/policy file.

#####################################################################
#ACTION SOURCE DEST PROTO
#
SECTION NEW
# Drop and log packets that come from the outside but pretend
# to have a local address
DROP:info net:192.168.0.0/24 all
DROP:info net:192.168.254.0/24 all
# Redirect incoming traffic to the correct server for WWW and email
DNAT all dmz:192.168.254.20 tcp www
DNAT all dmz:192.168.254.10 tcp 110
DNAT all dmz:192.168.254.10 tcp 143
DNAT all dmz:192.168.254.10 tcp 25

In its most basic form, what we’ve just defined here is that we want all traffic from anywhere destined for port 80 (www) to be sent to our win server.
All mail traffic, POP3 (port 110), IMAP (port 143) and SMTP (port 25) is to be redirected to our linux server in the DMZ.

There are a few more useful rules that we can include, for instance, I want to be able to access my servers through either ISPs from home (IP 123.45.67.89) and disallow everyone else from accessing it.

When I SSH to 30.40.50.62 or 100.90.80.70, on the normal port 22, I will access the firewall.
Now if I SSH to the non-standard port 2222, I will instead access the linux server.
Ports 5901 are for remoting through VNC on the linux machine, and port 3389 will be used for Remote Desktop connections to the win server.

To make sure my machines are up and running, I like to be able to ping them:

Note that ping will only work between the LAN and the DMZ and pinging my firewall from the Internet will result in the requests being silently dropped.
I usually prefer that configuration as it makes discovering the servers by random bots slightly less likely.

There are lots of other cool things we can do with forwarding but that will do for now.

shorewall.conf

The last file we’re going to look at is the main configuration file for shorewall.
See details about each option from the man page for shorewall.conf.

Most options are OK by default. The only ones that I have had to change are:

The first option tells shorewall that we want it to start automatically when the system boots.
That’s not enough though, so make sure that the service will be started:

# chkconfig shorewall --levels 235 on

Installing our firewall rules

Shorewall configuration files need to be compiled without error before the firewall is actually loaded by shorewall.
The command:

# shorewall restart

will stop and recompile the current configuration.
If there are any errors, the current firewall rules will be unchanged.
There are lots of other commands that can be issued. Check the man page for a complete list.

If you use PPPoE, you will want the firewall to be restarted every time the line reconnects.
The simplest way is to create a file /etc/ppp/if-up.local with only a single line:

shorewall restart

DNS

There is one remaining issue with our firewall: if a user on the LAN attempts to access the web server by its name the request will probably fail.
Same for accessing our mail server: we can configure our desktop to connect to 192.168.254.10 to get and send emails, but on the laptop we would usually use something like pop.acme.com instead so we can read our emails from outside the office.

Similarly, trying to access www.acme.com hosted on the win server from the linux server will fail.

One solution is to route traffic through the firewall but that’s actually fairly complicated to setup properly.
The shorewall FAQ 2 discourages this and instead recommends the use of split-DNS: it’s very easy to setup and it works like a charm.

dnsmasq

Just install dnsmasq on the firewall. There are ready-made packages available for it and a simple yum install dsnmasq should suffice.

Dnsmasq provides a simple DNS forwarding and DHCP service. I had already configured dhcpd -which is already fairly simple to configure- on my firewall so I won’t need DHCP from dnsmasq but you can easily set it up if you want.

On the DNS side, dnsmasq can be told to first try to resolve hostnames by looking at the standard /etc/hosts file and then query the DNS servers defined in /etc/resolv.conf if necessary.

This simple trick means that we can:

Keep our normal DNS service pointing to say 100.90.80.70 for www.acme.com so that people on the Internet will properly resolve their web requests to our win server.

Add an entry in the firewall’s hosts file to point local clients to 192.168.254.20 instead.

DNS resolution

There may be one last issue with DNS: in your /etc/resolv.conf you will have listed the DNS servers of one or both of your ISPs.
The problem is that some ISPs don’t allow access to their name servers from a network different than theirs.

The result is that each time any of the systems issues a DNS request it may fail and need to be sent to the next server instead, which may also fail and introduce delays in accessing named resources on the Internet.

One easy way out is to not use the ISPs DNS servers but instead only list the free OpenDNS name servers in your resolv.conf:

search acme.com
nameserver 208.67.222.222
nameserver 208.67.220.220

Then make sure that you disable DNS in your /etc/sysconfig/network-config/ifcfg-XXX configuration file for your PPPoE connection:

PEERDNS=no

Failure to do so will result in your /etc/resolv.conf file being rewritten with the DNS servers of one of your ISP every time you reconnect to them.

DHCP configuration

If you use dhcpd for local users, then you will need to make sure that its DNS server is set to the firewall’s:

On your local machines that use DHCP, make sure to renew your IP.
All other machines should be configured to use 192.168.0.1 as their unique DNS server and the machines in the DMZ should have their DNS set to 192.168.254.1.

Unless you reboot, don’t forget and flush the local DNS cache of each machine:
On Windows, from the command line:

C:\> ipconfig /flushdns

On Mac, from the terminal:

bash-x.xxx$ dnscacheutil -flushcache

Initial conclusions

I believe this type of firewall setup is fairly common and I hope that the -rather long- article helped you get your own setup in place.
In the -much shorter- follow-up articles, we’ll make our system as redundant as possible so our web and email services stay online even when one of the broadband connections fails.

In the meantime, don’t hesitate to leave your comments and corrections below.